The Domain Name System (DNS) is a name-resolution database most commonly
associated with the Internet. It was first defined as a way to replace the aging
HOSTS file system, which is explained in the next section. With Windows 2000,
Microsoft has made DNS the primary method of name resolution for Active
Directory (AD) networks. In fact, DNS is a required element for installing
ADso much so that the process of upgrading a member or standalone server
to an AD domain controller (DC) automatically installs the DNS server service if
you do not have a valid DNS server for AD to use.

We'll briefly touch on how and why DNS was created, because without that
background, troubleshooting DNS by testing with alternative name-resolution
methods won't fully make sense.

CAUTION

You will need to know how to install, configure, and troubleshoot the DNS
server service.

Brief History of DNS

As mentioned previously, DNS has its origins in the Internet. When the
Internet was small, every single TCP/IP (Transmission Control Protocol/Internet
Protocol) host on the Internet had a file called HOSTS that contained mappings
of every other host on the Internet and its IP address (hence, the name).
Whenever a new host (such as a workstation or server) was added to the Internet,
a new master HOSTS file was created and posted, and everyone on the Internet
would download the updated file. This was fine when the Internet was small and
changes were infrequent. However, as the Internet grew, the process of updating
HOSTS files on every system became increasingly unmanageable. Enter DNS. DNS was
conceived as a hierarchical namespace that allows the management of the Internet
namespace to be partitioned and distributed. As such, not every system needs to
know the name and IP address of every other system on the Internet.
Conceptually, the DNS hierarchy looks like a tree. At the very top is what is
known as the root domain, which is represented by a period (.). Below the
root domain are the top-level domains, which are the .com,
.net, .edu, .org, and so on that we are all familiar
with.

Below the top-level domains are the second-leveldomains, which
are what we work with every day when sending email or visiting a Web site.
Microsoft.com is a second-level domain, as is Army.mil and
Harvard.edu. When you visit
http://www.Inside-Corner.com,
you are accessing a host computer called www in the
Inside-Corner.com second-level domain.

Fully Qualified Domain Names (FQDNs)

With DNS, another important term to understand is the fully qualified
domain name (FQDN). This refers to the complete, unambiguous name of a host.
The FQDN contains everything from the host name through the root domain. An
example is the
http://www.microsoft.com.
FQDN. In this example, www is the host, microsoft.comis the second-level domain, com is the top-level domain, and the
trailing period represents the root domain. This FQDN is said to be
"unambiguous" because it uniquely defines a single host on the
Internet.

Relative Distinguished Names (RDNs)

Unlike an FQDN, a relative distinguished name (RDN) is just the part
of the host name that represents the host system. In the previous example,
www would be the RDN. These types of names are not used on the Internet
because of the likelihood the name would be ambiguous and unable to be resolved
to an IP address. However, RDNs are common on internal networks, because
corporate DNS servers check their local zones first to resolve a name (more on
zones later).

Now that we have explored some background information, let's examine DNS
as it relates to Windows 2000 and AD.